Interruption in a vacuum circuit breaker is achieved by vacuum interrupters which require only a short contact gap for circuit interruption. Vacuum interrupters typically comprise an insulating cylindrical envelope composed, for example, of glass or alumina, with endcaps mounted at each end to form a closed cylinder. Stationary and movable current conducting terminals are mounted through the endcaps at opposite ends of the cylindrical envelope. The movable terminal is moved axially to make and break contact with the stationary terminal contact, thereby making and breaking the electrical circuit. To permit axial movement of the movable terminal, it is necessary to provide a cylindrical bellows which is attached to the endcap at one end and to an internal portion of the movable terminal at the other end.
Vacuum interrupters of this type are required to maintain an internal vacuum on the order of about 10.sup.-7 Torr to interrupt the current flowing in the electrical circuit. Loss of vacuum permits air or other molecules to enter the evacuated volume, which reduces the internal vacuum and thereby reduces the interrupting capacity and dielectric strength of the internal vacuum. It is essential, therefore, that the interior volume of the interrupter remains hermetically sealed from the external atmosphere to maintain the internal vacuum. The quality of the vacuum interrupter components and materials must be consistently high to prevent introduction of contaminants into the internal vacuum, and to prevent the external walls of the interrupter from developing weak or porous areas which would threaten the integrity of the internal vacuum.
Suitable component materials have been developed and, in general, the interrupter components including the insulating cylindrical envelope, the current conducting terminals, and the endcaps are not prone to develop weak or porous areas which may threaten the integrity of the internal vacuum. One source of fatigue and, consequently, vacuum leakage is the brazed joints which hermetically seal the endcaps to the insulating cylindrical envelope. Another source of vacuum leakage is mechanical damage to the interrupter components which may occur during installation or adjustment of the vacuum interrupter in a vacuum circuit breaker.
FIG. 1 illustrates conventional prior art vacuum interrupter 10 comprising cylindrical insulating envelope 12 composed of alumina or the like, endcaps 14 and 16 mounted at opposite ends of insulating envelope 12, with stationary terminal 18 and movable terminal 20 mounted in opposite endcaps. Stationary terminal 18 is rigidly mounted through endcap 14, while movable terminal 20 is mounted through endcap 16 and guide 22 for axial movement along its central longitudinal axis. Cylindrical bellows 24 is hermetically sealed at one end to endcap 16 at brazed joint 26, and at the opposite end to an internal surface of the movable terminal at brazed joint 28. Bellows 24 permits axial movement of the movable terminal along its central longitudinal axis, and brazed joints 26 and 28 which provide hermetic seals maintain the internal vacuum during movement of the movable terminal.
The area most prone to mechanical damage which results in vacuum leakage has typically been the hermetic seals between the bellows and the endcap at joint 26 and between the bellows and the movable terminal at joint 28. The cylindrical bellows comprises a relatively thin, flexible material which is prone to fatigue and cracking under adverse conditions. The bellows and the sealing joints supporting the bellows in the vacuum interrupter may be damaged by axial overstress caused by excessive stroke lengths of the movable terminal, or by rotation or twisting of the movable terminal about its central longitudinal axis. Twisting of the movable terminal about its central longitudinal axis typically occurs during installation of a vacuum interrupter in a vacuum circuit breaker or similar protective device, as the interrupter is being positioned, adjusted, and finally fastened in place. Damage to the bellows due to such twisting motion generally occurs along joints 26 and 28, where the peripheral edges of the bellows are sealed to the endcap or the movable terminal.
Several techniques have been utilized to prevent damage to the bellows or its sealing joints due to rotation or twisting of the movable terminal assemblies of vacuum interrupters. Typically, the contact column of the movable terminal is provided with a non-round cross-sectional configuration for at least a portion of its length, and the external guide member is provided with a corresponding non-round bore to prevent rotation of the movable contact column in the guide member. This technique limits rotation of the movable terminal and the bellows, but it does not prevent end bearing friction which may occur when the entire interrupter is twisted and the non-round contact column of the movable terminal is forced against the corresponding walls of the guide member. This type of motion is particularly likely to occur during installation or adjustment of a vacuum interrupter in the vacuum circuit breaker, or removal of a vacuum interrupter from the vacuum circuit breaker.
Accordingly, it is an objective of the present invention to provide a vacuum interrupter having an internal volume which is hermetically sealed from the external atmosphere in a manner which substantially reduces contamination and leakage of the internal vacuum.
It is another objective of the present invention to provide an improved vacuum interrupter which is durable and reliable over the course of longterm operations.
It is still another objective of the present invention to provide an improved vacuum interrupter which reduces stress and fatigue at the brazed joints hermetically sealing the bellows at the movable terminal.
It is yet another objective of the present invention to provide a vacuum interrupter which permits rotation or twisting of an external portion of the movable terminal without transferring the stress and strain due to the rotation or twisting motion to the bellows.
It is still another objective of the present invention to provide an improved movable terminal assembly for use in a vacuum interrupter.